The present invention relates generally to a switching amplifier useful for driving a gradient coil in a magnetic resonance imaging (MRI) system and, more particularly, to such an amplifier which is capable of continuous arbitrary waveform generation.
Gradient coils in MRI systems require high, rapidly changing current as well as precise current control. To achieve fast current changes, a high voltage drive is needed, requiring high-voltage, high-current semiconductors (typically insulated gate bipolar transistors, i.e., IGBT's). Higher voltage devices generally have higher switching losses, limiting the maximum switching frequency attainable. With high switching losses, the time interval permitted for the high-voltage inverter to switch at a sufficiently high frequency to maintain precise current waveforms is also limited. A solution has been to add linear amplifiers, which, unfortunately, add relatively high losses. For trapezoidal coil currents, the high-voltage inverter provides the high voltage needed to obtain fast current rise and fall times; and during the flat top portion of the wave which does not require high voltage, a linear amplifier controls the current.
Unfortunately, using linear amplifiers as described hereinabove is not practicable for arbitrary current waveforms (i.e., that are not trapezoidal) because the losses are too great to allow such waveforms to be maintained for a sufficient time to permit high-performance imaging. Hence, it is desirable to provide a switching amplifier which reduces and distributes device switching losses such that arbitrary waveforms can be maintained for extended periods of time, thereby enabling high-performance advanced imaging.